3GPP (Third Generation Partnership Project) is currently in a process of studying use cases and identifying potential requirements for operator controlled discovery of and direct communication between mobile devices that are in close proximity. This activity is commonly known as “ProSe” (Proximity Services). It is expected that the corresponding technical stage 2 work on this topic starts soon when work on 3GPP Rel-12 begins. Possible use cases for direct UE-to-UE communication are for example: commercial use, social use, network offloading, UEs acting as mobile relays (e.g., for purposes such as coverage extension, mitigation of access technology mismatch, and alike), public safety, etc.
The “ProSe” topic can be divided into two parts:                Proximity Detection (this may be infrastructure assisted): This procedure determines whether given proximity criteria are fulfilled (“e.g. a UE (User Equipment) is in proximity of another UE”). The aim is twofold: to discover the proximity of two or more UEs in general and to clarify whether a direct UE-to-UE communication is possible. Criteria can be different for discovery and UE-to-UE communication.        UE-to-UE Communication (this may be under continuous control of the Mobile Network Operator (MNO)).        
Direct UE-to-UE Communication is also referred to as “Device-to-Device Communication” or “D2D Communication”. There are in principle two alternatives to realize such a direct communication path between mobile devices: the D2D air interface (called Ud in this description) could be realized by some type of short range technology in frequency bands different from the frequency bands used by the cellular network, such as e.g. Bluetooth or WiFi, or by re-using the LTE technology, which may be using the same frequency bands for the D2D communication as used by the cellular network.
The mobility of D2D devices that are using direct communication links as described above is limited to the coverage area of the direct communication link. This is typically some 100 meters. If the distance of the devices will exceed the coverage area due to the devices' mobility, the D2D connection will break.